Voting up is just one of a number of privileges that you can earn As a SO member. It's one of the first and easiest privileges to earn — you can earn 20 rep points by getting an answer voted up twice, and you get 15 for three upvotes on questions. Other privileges, like editing other people's questions or answers, require more trust, so you need more reputation before you're allowed to do them.

In short, you have to show at least a little interest in being a SO member before you get to vote.